The invention relates to an expansible fixing plug.
Such expansible fixing plugs are known per se. They comprise a fixing plug sleeve, into which an expansion screw can be screwed, wherein the expansion screw, as it is screwed in, widens the fixing plug. Normally, the fixing plug sleeve has longitudinal slits or slits running at an angle to the lengthwise direction, which divide the fixing plug sleeve into expansible tongues, which are expanded by screwing in the expansion screw. A further possibility is to manufacture the fixing plug sleeve from a resiliently and/or plastically expandable material, especially from a plastics material. As a consequence of the widening or expansion, the expansible fixing plugs are anchored in a drilled hole, for example in masonry.
The known expansible fixing plugs have the drawback that they become loose when the drilled hole becomes enlarged, for example, as a consequence of cracks developing in the masonry, and their anchoring or withdrawal force decreases.
The invention is based on the problem of constructing an expansible fixing plug of the kind mentioned in the introduction in such a way that it has a high anchoring or withdrawal force when the drilled hole becomes enlarged.
That problem is solved in accordance with the present invention. The expansion screw of The inventive expansible fixing plug has a screw thread, the rear flank of which in the screwing-in direction runs at a shallow angle of about 45xc2x0 or less to the axial direction of the expansion screw. The flank of the screw thread at the rear in the screwing-in direction forms an oblique or expansion surface, which additionally widens or expands the fixing plug sleeve when the expansion screw is subjected to axial loading in the withdrawal direction. The screw thread forms a cone-like expander body. If a drilled hole in which the expansible fixing plug according to the invention is anchored becomes enlarged, thenxe2x80x94when the expansion screw is loaded axiallyxe2x80x94the expansion screw is pulled a little way axially out of the fixing plug sleeve. As this happens, the flank of the screw is pulled a little way axially out of the fixing plug sleeve. As this happens, the flank of the screw thread at the rear in the screwing-in direction standing at a shallow angle of about 45xc2x0 or less to the axial direction expands the fixing plug sleeve further, the expansible fixing plug is expanded further remains anchored in the drilled hole with more or less unchanged anchoring force.
The expansible fixing plug according to the invention has the advantage that it has the ability to expand subsequently and is therefore suitable for use in zones subject to tensile forces. A further advantage is a high anchoring force also in a drilled hole that has not become enlarged, and an advantageous ratio between screwing-in torque of the expansion screw necessary to expand the fixing plug sleeve and the anchoring force of the expansible fixing plug in the drilled hole.
Furthermore, the expansion screw of the inventive expansible fixing plug has a core diameter that increases over the length of its screw thread from the front to the rear. The core diameter can increase in steps or gradually. The rearwardly increasing core diameter has the advantage that the screw thread is able to accommodate a relatively large transverse force (bending and shearing stress) in the rear region. The expansion screw is therefore capable of supporting a relatively large load suspended on its rear end and subjecting the expansion screw to transverse load. In addition, a relatively large tensile stress on the expansion screw is also possible, of course, which decreases towards the leading end over the length of the screw thread. Furthermore, the screw thread can be formed right up to the rear end of the expansion screw without sharply reducing the ability of the screw thread to withstand stress as a consequence of a reduction in cross-section of and a stress concentration in of the screw thread. This has the advantage that the fixing plug sleeve can be expanded and subsequently expanded by means of the expansion screw close to a surface, for example of masonry, in which the expansible fixing plug according to the invention is anchored.
In a preferred construction of the invention, the screw thread has a saw-tooth profile, the shallower and longer flank of which, forming the expansion surface, is directed rearwards in the screwing-in direction of the expansion screw. This construction of the invention exhibits good screwing-in behaviour of the expansion screw into the fixing plug sleeve. A further advantage is a large expansion area, formed by the thread flank at the rear in the screwing-in direction, in relation to the axial length of the screw thread.
In a preferred construction of the invention, the core diameter at the rear end of the screw thread corresponds approximately to the diameter of the threadless screw shank adjoining the rear end of the screw thread. In this way, a substantially progressive transition from screw thread to screw shank is achieved. A cross-sectional weakening or stress concentration at the transition from screw thread to screw shank is largely avoided in this construction of the invention. This is particularly advantageous because the transition from screw thread to screw shank is normally located close to a surface, for example of masonry, in which the expansible fixing plug is anchored. Transverse stress on the expansion screw is greatest at the surface of the masonry.
In one construction of the invention, the angle at which the flank of the screw thread at the rear in the screwing-in direction runs is about 28xc2x0 at the leading end of the screw thread. This produces an angle of taper of approximately 56xc2x0, angle of taper referring to the angle of the flank at the rear in the screwing-in direction at opposite points on the screw thread. The angle at which the flank at the rear in the screwing-in direction runs decreases towards the rear end of the screw thread to about 7xc2x0, which corresponds to an angle of taper of about 14xc2x0. The said angles produce a good ratio between axial force exerted on the expansion screw in the withdrawal direction and expansion force exerted by this force from the expansion screw on the fixing plug sleeve.
If the expansion screw is used in combination with a fixing plug sleeve manufactured from a relatively hard plastics material, it is advantageous to give the shallower and longer thread flank of the screw thread a concave form. The point angle of the thread therefore becomes somewhat smaller, so that the thread cuts its way more easily into the material. Furthermore, the run-out of the thread flank becomes shallower by virtue of the concave construction, which reduces friction at the start of axial displacement of the expansion screw in the fixing plug sleeve for subsequent expansion of the expansible fixing plug and increases it only as the axial displacement progresses. A subsequent-expansion behaviour in which the expansion force of the expansible fixing plug increases progressively with axial displacement is thereby achieved. A further advantage of a concave flank of the screw thread at the rear in the screwing-in direction is a reduced stress concentration by virtue of the relatively small flank angle at the root of the thread.
To create the necessary axial ability to move of the expansion screw for widening and subsequent expansion of the expansible fixing plug, in one construction of the invention the fixing plug sleeve has a yield and/or rupture point, which enables the fixing plug sleeve to elongate axially or to separate into two parts when the fixing plug sleeve is subjected to axial loading. If the fixing plug sleeve of the expansible fixing plug according to the invention is axially loaded, it elongates axially or is separated into two parts. When subjected to axial loading, the rear part of the fixing plug sleeve moves axially, together with the expansion screw located therein, a little way away from a front part of the fixing plug sleeve anchored in the masonry. This displacement causes the expansion screw to expand the fixing plug sleeve in its front region, as described above, and thereby anchor the expansible fixing plug in the drilled hole. The described axial movement of the rear part of the fixing plug sleeve jointly with the expansion screw located therein with respect to the front part of the fixing plug sleeve anchored in the masonry also occurs when the drilled hole becomes enlarged, for example, as a consequence of cracks developing in the masonry, so that the expansible fixing plug according to the invention expands subsequently when the drilled hole becomes enlarged. As already mentioned, the anchoring or withdrawal force of the expansible fixing plug according to the invention remains more or less unchanged by this subsequent expansion when the drilled hole becomes enlarged.
A yield and/or rupture point are not necessarily alternatives, they may be one and the same. When the expansible fixing plug is subjected to axial loading, the yield and/or rupture point can initially allow an axial elongation of the fixing plug sleeve. If the axial loading increases, the fixing plug sleeve separates into two parts at the yield and/or rupture point. The yield and/or rupture point can be formed, for example, by a reduction in the wall thickness of the fixing plug sleeve, or by providing a circumferential or alternatively a helically running groove on the fixing plug sleeve.
The construction of a yield and/or rupture point on the fixing plug sleeve of the expansible fixing plug according to the invention has in particular the advantage that the expansible fixing plug can be introduced, for example, hammered, into a drilled hole with the expansion screw already located in the fixing plug sleeve. It is not necessary first of all to introduce the fixing plug sleeve into the drilled hole without the expansion screw and then to screw in the expansion screw, the operation of screwing in the expansion screw can be omitted. The expansion screw widens and/or expands the expansible sleeve when subjected to axial loading in the withdrawal direction so that the expansible fixing plug according to the invention is anchored in the drilled hole by the loading. Expansion of the expansible sleeve by screwing in the expansion screw is superfluous. If required, after the expansible fixing plug has been introduced into the drilled hole the expansion screw located in the fixing plug sleeve can be briefly tightened to bring about widening and/or expansion of the fixing plug sleeve, and hence anchoring of the expansible fixing plug in the masonry, even with no loading on the expansible fixing plug.
Because it is possible to introduce the expansible fixing plug according to the invention with the expansion screw located in the fixing plug sleeve into a drilled hole, without screwing in the expansion screw, anchoring of the expansible fixing plug according to the invention is simplified and shortened. There is the added advantage that when the fixing plug sleeve is manufactured, for example, by injection-moulding, from plastics material, the expansion screw can be used as core. The expansion screw is placed as the core in an injection-moulding tool and the plastics material forming the fixing plug sleeve is subsequently injected around it. A separate core is therefore unnecessary, as it hollowing out of such a separate core from the fixing plug sleeve after injection-moulding and screwing the expansion screw into the fixing plug sleeve. The expansible fixing plug with the expansion screw, around which the fixing plug sleeve has been injected, is ready for immediate use after removal from the mould. A parting agent applied to the expansion screw prior to injection-moulding of the fixing plug sleeve acts as a lubricating layer, which ensures easy-running axial displacement of the expansion screw in the fixing plug sleeve and hence ensures a good ratio of expansion force to axial loading.
In a preferred construction, the fixing plug sleeve departs in an expansion region from, for example, a cylindrical form with a notional straight axis; at least in the expansion region, the fixing plug sleeve has, for example, an undulation in the lengthwise direction or a helical form. Expressed in more general terms, a surface of the fixing plug sleeve in the expansion region runs in the lengthwise direction at an angle to a notional cylindrical surface or to the longitudinal axis of the fixing plug. At the same time, the angle at which the surface of the fixing plug sleeve runs to the longitudinal axis of the expansible fixing plug changes over the length of the fixing plug sleeve. The purpose of this construction of the invention is to render the fixing plug sleeve deformable in a transverse direction in the expansion region. As it is driven into a drilled hole, the fixing plug sleeve is deformed in the sense of being straightened out; for example, when the fixing plug sleeve is of undulating construction, the crests of the undulation are pressed radially inwards. The fixing plug sleeve is consequently located under bias in the drilled hole.